This invention relates generally to the art of polymers and more particularly to the art of polymeric packaging film.
Polymeric films are ideally suited for packaging materials because of ease of fabrication thereof and because of the ability of some polymeric materials to maintain a packaged product in a desirable or unaffected condition. Particularly associated with the preservation of food articles is the ability of many polymeric films to resist the permeation of deleterious atmospheric gases into the contained product. One particular material which has been successful for packaging food articles is a copolymer of vinylidene chloride with one or more other monomers usually vinyl chloride. This copolymer is normally referred to commercially as saran. Saran has many characteristics, however, which make it a less than totally satisfactory packaging material. The chief disadvantage of saran is a difficulty associated with the forming of film products from resin melts.
A proposed replacement for saran as a commerical material possession resistance to oxygen permeation is the hydrolyzed product of ethylene and vinyl ester. Since the hydrolysis is normally greater than 96 percent complete, the hydrolysis is product is essentially a copolymer of ethylene and vinyl alcohol. Copolymers of ethylene and vinyl alcohol an processes for producing them are fully described in U.S. Pat. Nos. 2,386,347 and 3,585,177, herewith incorporated by reference. The process for forming such a composition into a film is described by C. H. Iba et al in U.S. Pat. No. 3,419,654 also herewith incorporated by reference.
Films composed of copolymers of ethylene of vinyl alcohol have been found to have a permeability to oxygen which is comparable to that of commercial saran while not possessing any halide moieties within the polymer structure.
A problem, however, with copolymers of ethylene and vinyl alcohol, while possessing excellent low oxygen permeability characteristics, is the effect that moisture has upon the desirable properties of the copolymer. Copolymers of ethylene and vinyl alcohol are soluble and thus swollen to some extent in water and are affected by moisture such that while retaining dimensional integrity the resistance to oxygen transmission is severely diminished. Thus any use of such copolymers for an application where low oxygen permeability is required must include measures to prevent moisture from coming into contact with the copolymer and must totally preclude the use of such copolymer in direct contact with a moisture bearing food article.
An additional problem with copolymers of ethylene and vinyl alcohol is that, while possessing low oxygen permeability, the material is extremely stiff having a high modulus of elasticity. This characteristic is rather irreconcilable with the deleterious effect which moisture has upon the oxygen transmission characteristics in that the layer of ethylene and vinyl alcohol copolymer is preferably made thick in order to offset the deleterious effects of moisture while desirably made thin in order to preserve flexibility. A thick layer does not possess the desirable flexibility required of packaging containers.